you wont be able to control it via RF.   you may be able to get some type of IR controlled relay, and use that to act like a wall switch.   i suppose you could even use an ir controlled relay, and solder to the contacts of the fireplace rf remote, simulating a button press on the rf remote.

before you get into too much of a science project though, i would contact the fireplace manufacturer and see if there are other control options.  maybe you can get a part that will add ir control.